The Virtual Lab (VL) project's initial goals were to
provide Web access for electromagnetic scattering and radiative transfer simulation
applications developed at the DLR's Remote
Sensing Technology Institute and to make them more accessible for technology
transfer and scientific exchange. To reduce perprogram development effort, it
became clear early on that providing Web interfaces for a substantial selection
of programs would only be feasible with a generic platform.

The VL
hardware currently comprises a heterogeneous cluster containing seven Intel/Linux
machines and one Sparc/ Solaris machine located at DLR's
site in Neustrelitz. The VL's
main implementation language is Python
along with Zope, MySQL,
and OpenLDAP. The project
is based on first- and second-generation Web technology, nb. HTML 3.2 and HTTP
1.0.

Remote sensing of the earth and its environment
becomes more and more important in the context of sustainable development and
global change monitoring. Common to all remote sensing techniques is the fact
that they are based on information coming from scattered and transmitted electromagnetic
waves in different spectral ranges. Therefore, realistic models of how such
electromagnetic waves are transmitted and scattered under certain conditions
are of particular importance in remote sensing applications.

During the past we have developed several
programs allowing light scattering analysis on dielectric but, in general, nonspherical
particles in the resonance region. In this region the scattering models must
be based on a full-wave analysis of Maxwell's equations. This is a mathematically
pretentious task. Concerning radiative transfer we have developed models to
perform simulations in the infrared (line-by-line model) as well as the visible
and the ultraviolet spectral range. All these programs have been successfully
applied to retrieve trace gases in the earth's atmosphere, to determine cloud
and aerosol properties, and to perform system studies for the detection of natural
and anthropogenic high-temperature events like volcanoes and bio-mass burning.
Our scattering and radiative transfer programs represent the state-of-the-art
in many aspects, and they are also of interest in other fields than remote sensing
(in technical and medical diagnostics, for instance)

In the VL,
sophisticated programs are available allowing light scattering analysis up to
the geometric optics region on various classes of nonspherical particles such
as spheroids, hexagonal and irregular ice particles, and Chebyshev-like particles
which are used in remote sensing to model aerosol components, microphysical
properties of Cirrus clouds and hydrometeors, for instance. Amongst other, the
following programs can be used:

mieschka: This program calculates the integral and differential scattering properties of axisymmetric
particles in a fixed orientation. It includes spherical particles as a special
case.

pmieschka: This program calculates the integral and differential scattering properties of randomly
oriented axisymmetric particles. It also includes spherical particles as a
special case.

CYL: The program CYL aims to compute integral and differential scattering quantities of finitely
extended cylinders with circular, hexagonal or octagonal cross sections in
random orientation.

QCACP: The program
QCACP aims to model an inhomogeneous host scatterer which contains up to three
different classes of densly packed small spheres. The inhomogeneous host scatterer
is replaced by an homogeneous analog with an effective permittivity. The method
is based on the so-called "quasi-crystalline approach with coherent potential
(QCA-CP)" [Tsang et al., 1985].

The methodological background of mieschka,
pmieschka, and CYL are
the generalization of the separation of variables method [Rother,
1998] (GSVM) applied in spherical coordinates for the first two programs.
In CYL this method is applied in cylindical coordinates
in combination with Huygen's principle to find an approximation for finite cylindrical
columns having noncircular cross-sections [Rother
et al., 1999]. Essential numerical simplifications can be achieved
if the scattering geometry exhibits a certain symmetry. This was demonstrated
in Rother et al. [2001], for instance.

There are two general approaches which have been widely used in the past to
solve light scattering problems on nonspherical particles rigorously. These
are the Finite-Difference methods (FD) and the Boundary Integral Equation methods
(BIE) which have traditionally been treated separately. FD methods are based
on the description of the scattering problem in terms of partial differential
equations. They apply a discretization scheme to some (Method of Lines (MoL)
[Rother, 1999a]) or to all spatial coordinates
(conventional FD methods, see e.g. Taflove [1995])
of the corresponding partial differential equation. BIE methods, such as Waterman's
T-matrix approach, start from Green's theorem in conjunction with the Helmholtz
equation and the related free-space Green's function [Tsang
et al., 1985]. With the GSVM both approaches can be condensed into
a common mathematical body [Rother, 1999b].
This is achieved by deriving the surface Green's function related to the scattering
problem. This Green's function establishes the link between BIE methods and
methods based on the solution of the corresponding partial differential equation.

Modelling the transfer of electromagnetic radiation in the atmosphere is important
for meteorology, climatology, and atmospheric remote sensing. Radiative transfer
in the atmosphere is driven by absorption, emission, and scattering of light
at molecules, aerosols, and hydrometeors [Liou,
1980]. The change of radiation intensity (radiance) passing through the
atmosphere is formally described by a complex equation which does not allow
for a general solution. A large variety of radiative transfer codes have been
developed in the past decades based on approximations etc. appropriate for a
a certain application and/or wavenumber regime.

The following models are currently available in the VL for radiative transfer:

MIRART: The Modules for InfraRed Atmospheric Radiative Transfer are a suite of programs for high-resolution,
line-by-line radiative transfer calculations emphasizing efficient and reliable
numerical algorithms and a modular approach appropriate for simulation and
retrieval in atmospheric spectroscopy [Schreier,
2003]]. The code has been carefully tested in the framework of two extensive
intercomparisons [Clarmann, 2002]]. Recently,
the PyFort tool
has made individual MIRART modules accessible from a Python layer. These Python scripts are usually called from a Unix command line but
are also available through the VL.

Squirrl: (Schwarzschild Quadrature InfraRed Radiation Line-by-line)
SQUIRRL is the main program of the MIRART suite of programs.

PFUI: An earlier 'Python Fascode User Interface'
was an inspiration for the VL's
development [Schreier, 2001]. PFUI already
significantly simplified the usage of the Fascode (fast atmospheric signature
code), a line-by-line program that the US Air Force developed. Now the same
functionality is available in the VL's
unified platform. (Due to licensing issues, only input-file generation is
supported.)

WIMP -- Web Interface Modtran using Python
(same as for PFUI).

ST -- The radiative transfer code ST is designed
to calculate the full Stokes vector in an atmosphere surface system with one
homogeneous atmospheric layer including aerosols, Lambertian surface or a
wind roughed water surface. The matrix operator method is used to solve the
radiative transfer equation. Aerosols are considered as spheres. The output
is the upward and downward diffuse Stokes vector at the top and bottom of
atmosphere.

The following figures show an excerpt of a typical session using the MIESCHKA
code for computing scattering properties for spherical and non-spherical particles.
For other software components the sequence of providing the relevant input parameters
is similar.

Figure 1 shows the complete specification for
MIESCHKA to perform a single scattering calculation for a homogeneous water
drop. Before the required input data is complete, the user is requested to fill
out a sequence of individual HTML forms containing input masks.

As soon as the input for the numerical
experiment is complete, the "Start Task" button
will appear near the top of the screen.

Figure 2 indicates that the VL cluster has
completed this particular task. This screen also echoes standard output and
standard errors during the task.

After pressing the Show Results" button a plot pops up, Figure 3,
serving as a quicklook. This plot shows the scattering phase function for a
purely scattering water drop.

The complete results for each task are
packed into a tar-file "resultfiles.tar" which
can readily be downloaded to the user's local directory.

Using the button "Abort
task and go back to InputDialog" allows the user to modify some of the
parameters and re-run the task, leading to another plot, in this case for an
absorbing water drop, see Figure 4.